LIFE-HISTORY ADAPTATION ALONG A LATITUDINAL CLINE IN THE WATER STRIDER AQUARIUS-REMIGIS (HETEROPTERA, GERRIDAE)

We addressed the general hypothesis that life history differences among eastern populations of the North American water strider, Aquarius remigis (Heteroptera: Gerridae), along a north-south gradient are manifestations of genetic differentiation due to natural selection. We raised offspring of two field-caught populations from each of three latitudes in a common laboratory environment at 20 degrees C and two photoperiods. Nearly all Quebec (PQ) individuals (ca. 46 degrees N) entered diapause to reproduce the following spring (univoltine life cycle), while intermediate proportions of New York (NY; ca. 43 degrees N) and New Jersey (NJ; ca. 41 degrees N) individuals reproduced directly, producing a second generation (bivoltine life cycle). PQ females were smaller, developed faster, and laid smaller eggs than NY and NJ individuals; NY and NJ populations differed little in these variables. NY females had longer life spans than either PQ or NJ females, but lower oviposition rates. Total reproductive output did not differ across latitudes. Photoperiod affected body length, development time, and reproductive pathway, resulting in a latitutide by environment interaction. PQ individuals reproduced directly under 15L:9D (summer) conditions only, while the NY and NJ populations exhibited more direct reproduction under 13L:11D (spring or fall) conditions. Some life history characters of the NY and NJ populations displayed the higher variability indicative of phenological transition zones. These results indicate local adaptation of populations to long-term climatic patterns. Water striders appear to adapt to longer seasons by extending development, growing larger, and breeding directly. Larger body size and extended or rapid development are associated with bivoltinism and increases in egg size, but not necessarily with higher fecundity or oviposition rate. The phenological transition zone appears to be unrelated to a transition zone a little further south established by allozyme data and morphology, as all populations studied here could be electrophoretically identified as the northern ''type''.